Mobile radio communication systems provide for two way radio communication between a central control station and a fleet of cars, trucks, or other vehicles. Typical users of mobile radio communication systems include, for example, police departments, fire departments, taxi services and delivery services. Present mobile radio communication systems can be configured to provide for radio communications between the control station and all vehicles in a fleet, between the control station and selected vehicles in the fleet, or between different individual vehicles or groups of vehicles within a fleet.
Conventional mobile radio communication systems are typically organized with a number of vehicles and a control station assigned to a single common channel for a given coverage zone. A user assigned to the common channel must wait until no one else is transmitting on that channel before sending a message, because only one transmission at a time can be processed by a single channel. Even when a transmission is addressed to only one user in a conventional system (and therefor not heard by other users), the other users in the system must wait until that transmission is completed before they can use the system to communicate.
Mobile radio communication systems using transmission trunking are an improvement over conventional mobile radio communication systems in that trunked systems enable two or more users to communicate on the system at the same time. For instance, the dispatch console in a transmission trunked communication system can be communicating with one of the vehicle in the fleet; while, at the same time, two other vehicles in the fleet can be using the same trunked system to communicate with each other. Transmission trunked communication systems group a number of channels for the same coverage zone together into a single radio communication system, with each channel accessible to each user in the system. Because each user will only need to communicate over the trunked system part of the time, and because it is unlikely that all users will need to communicate at precisely the same time, the number of channels assigned to a trunked system group can always be less than the number of users allocated to that trunked system group.
The United States Federal Communications Commission (FCC) has assigned 600 channels in the 800 MHz band for trunked land mobile use. Each channel is comprised of a pair of assigned frequencies, a mobile transmit frequency and a repeater transmit frequency. Mobile transmit frequencies are 806-821 MHz, with the repeater transmit frequencies exactly 45 MHz above the corresponding mobile transmit frequency, or 851-866 MHz. Channel spacing is 25 KHz, with the maximum allowed deviation between channels being +/- 5 KHz. In September 1987, the FCC also allocated 399 channels in the 900 MHz band for trunked land mobile use. Mobile transmit frequencies are 896-901 MHz, with the repeater transmit frequencies exactly 39 MHz above the corresponding mobile transmit frequency, or 925-940 MHz. Channel spacing is 12.5 KHz, with the maximum allowed deviation between channels being +/- 2.5 KHz.
In a transmission trunked communication systems, a signaling protocol is used to send and receive control signals among users on each channel in the trunked system and a switching protocol is used to establish which channels those users will be communicating over. The preferred conventional transmission trunked communication system uses a signaling protocol that transmits the control signals in the subaudio band simultaneously with the transmission of voice or data information signals. Signaling protocols that can communicate control signals within the constraints of the subaudio band are preferred, because use of the subaudio band precludes the need for using a dedicated channel for transmitting the control signals (thereby reducing the number of available channels for voice and data communications). the switching protocol is used by the trunked system to automatically find and engage an open channel when a user initiates a transmission. To maximize the trunking capabilities of such a system, the switching protocol must efficiently allocate channels in the trunked system and avoid channels that are already in use at the time the transmission is initiated. For further explanation of the preferred conventional transmission trunked communication systems, reference is made to the description of the operation of the ClearChannel LTR.RTM. system contained in the manual entitled "E. F. Johnson ClearChannel LTR Application Note". Part No. 009-0001-020 (Rev. 5, Oct. 1988, available from E. F. Johnson Company, Waseca, Minn.
Transmission trunked communication systems have proven to be an economical and effective means for establishing voice and data communications between a dispatch console or control station and a fleet of mobile vehicles in a given coverage zone. However, the capability of such trunked systems to provide radio communications over a wide area serviced by a plurality of preferably adjacent coverage zones has been limited because of the problems involved in linking multiple coverage zones and because of the limitations of the switching and signaling protocols of present transmission trunked communication systems.
Present transmission trunked communication systems of the type described above are generally unable to transfer voice/data communication between coverage zones, because the repeaters in such systems are interconnected only by a single time slot status bus. As a result, the only method of interconnecting repeaters in different coverage areas is to use an external network, i.e. routing the communication as a long-distance telephone call from a repeater interconnect or interface to a telephone exchange and then back through a second repeater interconnect to the remote repeater location. These channels are then "hung" to lock the channels in for the duration of the call. In addition to the increased expense and inconvenience of such an external network, the use of a repeater interconnect prevents usage of the channels in each trunked system over which the communication is occurring for the entire period of the communication.
Even in those instances where inter-coverage zone communications are made through an external network, such communication are private for individuals calls and are limited to a few preselected users on each system. It would be desirable to provide unique ID numbers for each user on a wide area network to allow for direct entry of the unique ID to establish private communication between users on the network. Unfortunately, it is impossible to accommodate unique ID numbers in the preferred present transmission trunked communication system because the number of digital bits required to represent such unique ID numbers exceeds the maximum number of bits that the trunked systems can communicate via the established signaling protocol. The signaling protocol of the preferred transmission trunked communication system is also limited in the number of channels that may be grouped together per trunked system. For example, the signaling protocol of the LTR.RTM. trunked system is limited to 20 channels per system and uses a look-up table to translate the channel information transmitted by the signaling protocol into the actual frequency pair assigned to that channel.
Another problem with the conventional switching and signaling protocols is that such protocols are unable to allow for the implementation of an extended feature set of radio communication capabilities on the preferred conventional transmission trunked communication systems. For example, present trunked systems are generally unable to establish priority access for users in a given coverage zone. Present trunked systems are also unable to interrogate and reprogram or otherwise modify the operation of mobiles without having the mobile transceiver unit physically brought to a service facility to be reprogrammed. A mobile traveling from one coverage zone to another coverage zone, for instance, needs to be reprogrammed for a different set of groups over which communications will be received. Such reprogramming cannot be done "on the fly" in present systems, and mobiles are therefor not able to transit coverage zones at will.
The present invention is particularly concerned with two problems in conventional switching and signaling protocols: (1) the reliance on a single home channel over which the subaudio control signals are transmitted, and (2) the inability to scan the activity within a given coverage zone by monitoring the subaudio control signals on a single channel. In present LTR systems, each group of mobile radios is preprogrammed to monitor a single home channel frequency. If the repeater for the home channel frequency is disabled or otherwise fails, the entire group of radios monitoring that home channel will no longer receive any of the control signals transmitted by the system and all of the mobile radios monitoring that home channel would effectively be out of service. With respect to the second problem, often times it is desirable to allow a transceiver to scan several groups of ID's to monitor communication traffic over several channels. This process is commonly referred to as scan mode. In the present LTR systems, the transceiver must switch through all of the reception frequencies for the group ID's being monitored in scan mode by changing the reception frequency programming of the mobile transceiver. This continual changing of reception frequency significantly degrades the amount of time that the mobile transceiver may be monitoring the system for activity.
Although present transmission trunked communication systems have proven to be an economical and effective means for establishing voice and data communications between a control station and a fleet of mobile vehicles in a given coverage zone, it would be advantageous to provide a method and apparatus for establishing an alternate home channel for a land mobile transmission trunked communication system that includes switching and signaling protocols especially adapted for communication of control signals in the subaudio band over both the home channel and the alternate home channel as the single alternate home channel for all channels within a given coverage zone.